1. Summary

Traffic of the Japanese Naval Attache in Berlin was of special value to radio
intelligence because it gave full details on technical matters under investigation
by the German navy, and also gave an insight into the future broad policy
of the Naval High Command. A very large amount of information was furnished
on all sorts of technical matters, including U-boats, aircraft, weapons, radar,
radar defense, and the extent of Axis knowledge of Allied weapons, devices
and procedures. Of particular value to the Atlantic Section was the data on
the new type XXI, XXIII, and XXVI U-boats, with which the Germans hoped to
regain their former power on the high seas. With the early knowledge of the
main feature of the new types, their higher underwater speed, and subsequent
fuller details, it was possible to appreciate the proportions of the anticipated
new German measures. Of no less importance was the ability of radio intelligence
to follow closely the progress in construction of the new type boats.

2. The special significance of Berlin-Tokyo traffic

Traffic between the Japanese Naval Attache in Berlin and his superiors supplied
certain unique contributions to the general picture which were not available
in the administrative and operational traffic of the German navy. Whereas
the technical material sent in U-boat traffic was being passed to people who
already had a knowledge of the subject discussed through possession of handbooks
and other aids, hence presumed a prior knowledge of most details, the Japanese
were in much the same position as their enemies in that they had no such extensive
prior knowledge. Therefore, when a new device or measure was reported to Tokyo
by the Japanese Naval Attache, full details had to be given and were thus
made available to the Allies through decryption.

The information to be gained form Japanese Naval Attache and diplomatic traffic
was also valuable because, in addition to its full discussion of technical
matters, it provided an insight into the line of major policy adopted by the
German navy after the U-boat was driven from the North Atlantic in the spring
of 1943. The type XXI U-boat, which was to be the

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mainspring of that policy, will be discussed at some length in following
paragraphs. The selection of new type U-boats as a prime example of the value
of Japanese Naval Attache traffic should not, however, lead to disregard of
the importance of the information furnished. Of direct Naval interest were
details of midget, transport and mine-laying U-boats; new torpedoes, including
the turbine torpedo still under development at the time of the German surrender;
and location and anti-location devices. Of a more general interest were the
details of radar and radar defense; jet-propelled and other aircraft; V-weapons
and many other types of enemy weapons; and the extent of enemy knowledge of
Allied weapons, devices and procedures.

3. Allied interest in the effort toward "Total Underwater Warfare"

It was apparent to the Allies as well as to the Germans that, after the utter
defeat to which the U-boat had been subjected in spring of 1943, the German
navy would be forced to turn to the skill of its technical departments even
more urgently than before. The situation demanded quick, effective and revolutionary
measures, and it was a matter of supreme importance for the Allies to find
out what those measures were to be in order to plan counter-measures before
any large measure of success could be achieved by the enemy. The introduction
of the Zaunkonig and the Schnorchel have already been discussed in other chapters.
They were part of the general plan to restore to the U-boat its old power.

It was known, however, that other plans were also in the making. On 6 March
1944 the Japanese Naval Attache reported to Tokyo:

Although German submarines were amazingly effective at first the
enemy now has invented new defensive weapons and in addition devised great
counter-measures which have brought about a continued decrease in the damage
inflicted by submarines. Consequently the German navy had determined to improve
the submarine and we believe they are making plans for the future, but judging
from what we know now, we do not expect any great activity form new type submarines
before autumn.

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The direction of German thought, as was to be expected
in view of past developments, and as was indicated by the introduction of
Schnorchel, was toward severing the tie which bound the U-boat to the surface
and made discovery by radar possible.

Schnorchel was a step in that direction, but only a step,
and it was feared by the Allies that exhaustive research might lead to a
U-boat which would not need to go to the surface or even near the surface
during the entire period of a cruise. It was thus a great relief to be able
to learn from reports of the Japanese Naval Attache that while new type
U-boats were indeed to be introduced, their chief advantage over older
types provided with Schnorchel would be in their increased underwater speed.

4. First details of the new type U-boats

On 28 December 1943 (in a message not read until June of 1944), the Japanese
Naval Attache in Berlin sent to Tokyo a description of two new types of
U-boats, details of which had not yet been officially released to him by
the German navy. The information represented the report of Naval Inspector
Tomonaga, who had been permitted to view plans and charts at the shipyard
and to catch a glimpse of part of one of the vessels. The Attache himself
had first viewed the plans the preceding August at the Deschimag Shipyard,
and his observations, added to those of the Inspector, provided specifications
and other information sufficient to indicate the trend in U-boat construction
for the remainder of the war. As was later confirmed by official information,
it appeared that the chief feature of the new U-boat would be their increased
underwater speed, which would be made possible by large-capacity batteries.

Two new types were described, the large type XXI, designed for long-range
operations; and a small type designed for coastal operations. The small
U-boat was referred to as type XVIIb but later evidence pointed to the conclusion
that the U-boat under construction was actually the type XXIII, concerning
which a large amount of information was later gained from Attache traffic.
As to the state of construction of the new boats, the Attache considered
that large numbers were already in the construction stage, would be completed
from about August or September. (JNA #479, 28 December 1943)

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5. Speeds

Since the increased underwater speed of the new type U-boats was to be
their chief advantage over older types, a dispatch in which pertinent figures
were given, Japanese Naval Attache #316 of 13 August 1944, was of more than
passing interest. The speeds and ranges quoted were as follows:

Type XXI(6 battery compartments)

Type XXIII

Speed(knots)

Range(miles)

Speed(knots)

Range(miles)

3

450

2

250

6

265

4

175

8

165

6

113

11

110

8

70

14

45

10

43

top

24

top

22

According to Japanese Naval Attache #420 of 12 September 1944, the tested
top speed for type XXI was about 16 knots, and it was hoped that this could
be increased to 17 or 18 knots. Also of significance was the type XXI's lowest
submerged speed on electric motors, 0.5-1 knot, at which the motor was said
to be noiseless as the result of reduction by rubber belts. For type XXIII
the tested top speed was 12.5-13 knots. It can be seen from the above table
that use of the higher underwater speeds would be severely limited by the
short ranges at which such speed could be maintained before exhaustion of
the batteries.

6. Delay in completion of the new type U-boats

The Japanese anxiety over the condition of German striking power led to
continual inquires as to when the new type U-boats would be ready for combat.
The serious construction difficulties due to Allied bombings and other factors
were reflected in repeated postponements in estimates of that date.
By the reading of Japanese messages it was possible for radio intelligence
to keep informed of construction developments. On 26 July 1944,
Ambassador Oshima reported concerning the type XXI boat:

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At present it is in process of construction and its effect will first be
felt from about September; and by November or December it is said that greater
results will be effected by its use in great quantity. (#756)

Again, on 12 August (#621), Dönitz was quoted as saying that the new offensive
U-boat warfare would begin "at one swoop" after a considerable number
of the new U-boats had been completed, therefore in the winter of 1944-45.
But on 3 December (#753) the Naval Attache wrote with regard to combat use
of new type U-boats:

Although the German submarine fleet and ministry of military supplies
are making feverish preparations, there will probably be no results until
about March of next year.

Even in March 1945, the best that could be reported concerning type XXI boats
was that they would be ready on a large scale in May or June (#301, 7 March).
Actually, only four of the type XXIII's made war cruises, while the only type
XXI to leave port for an operational cruise was U-2511 (Schnee) which was
outbound on the day of surrender.